1. Field of the Invention
The present invention relates to an image display device capable of displaying an image toward a plurality of points of view and a portable terminal device mounted with this image display device.
2. Description of the Related Art
Attempts have been made to develop an image display device capable of displaying different images toward a plurality of points of view. One example is a three-dimensional image display device. The three-dimensional image display device displays parallax images for the right eye and left eye, and a viewer looking with both eyes at the images differentiated between right and left can perceive a three-dimensional image.
To realize this function concretely, many three-dimensional image display systems have been studied. Three-dimensional image display systems can be broadly classified into two types, one using glasses and the other using no glasses. The former can be further subdivided into the anaglyph type utilizing differences in color and the polarization glass type. But both of them have the essential problem that the viewer must use glasses. For this reason, the latter, using no glasses, has become more popular, and this can be sub-classified into the parallax barrier type and the lenticular lens type.
First will be described the parallax barrier type. The parallax barrier type was conceived by Berthier in 1896 and verified by Ives in 1903. FIG. 12 is an optical model diagram illustrating a three-dimensional display method by the parallax barrier system. As shown in FIG. 12, a parallax barrier 105 is an optical barrier in which many vertical strip-shaped apertures, namely slits 105a, are formed. Near one surface of this parallax barrier 105, there is arranged a display panel 106. In the display panel 106, pixels for the right eye (hereinafter “right-eye pixels”) 123 and pixels for the left eye (hereinafter “left-eye pixels”) 124 are arranged in the direction orthogonal to the lengthwise direction of the slits 105a. Near the other surface of the parallax barrier 105, namely on the reverse side to the display panel 106, there is arranged a light source 108.
Lights emitted from the light source 108 are partly intercepted by the parallax barrier 105. On the other hand, a part of the lights having passed the slits 105a without being intercepted by the parallax barrier 105 passes the right-eye pixels 123 to become luminous fluxes 181 or pass the left-eye pixels 124 to become luminous fluxes 182. The position of the viewer to be able to perceive a three-dimensional image then is determined by the positional relationship between the parallax barrier 105 and the pixels. Thus, it is necessary for the right eye 141 of the viewer 104 to be within the area where all the luminous fluxes 181 matching a plurality of right-eye pixels 123 pass and the viewer's left eye 142 to be within the area where all the luminous fluxes 182 pass. The viewer can perceive a three-dimensional image when the middle point 143 between the viewer's right eye 141 and left eye 142 is positioned within the rectangular three-dimensional visible area 107 shown in FIG. 12.
The line segment which passes the intersection point 107a of diagonal lines in the three-dimensional visible area 107 is the longest, out of the line segments which extend in the arraying direction of the right-eye pixels 123 and the left-eye pixels 124 in the three-dimensional visible area 107. For this reason, since the tolerance for lateral deviations of the viewer's position is at its maximum when the middle point 143 is located at the intersection point 107a, this is the most preferable position of viewing.
Therefore, by this three-dimensional image display method, this distance between the intersection point 107a and a display panel 106 is regarded as an optimal viewing distance OD, and the viewer is recommended to watch an image at this distance. A hypothetical plane whose distance from the display panel 106 constitutes the optimal viewing distance OD in the three-dimensional visible area 107 is referred to as the optimal viewing plane 107b. This causes lights from the right-eye pixels 123 and the left-eye pixels 124 to reach the viewer's right eye 141 and left eye 142, respectively. As a result, the viewer is enabled to perceive the image displayed on the display panel 106 as a three-dimensional image.
For instance, Table 1 in Nikkei Electronics, Jan. 6, 2003, No. 838, pp. 26-27 (Reference 2) contains a cellular phone mounted with a 3D-compatible liquid crystal panel. The liquid crystal display panel constituting the three-dimensional image display device in this cellular phone measures 2.2 inches diagonally and has respectively 176 display dots horizontally and 220 display dots vertically. There is further provided a liquid crystal panel for the switching purpose to turn on and off the effect of a parallax barrier, permitting change-over between three-dimensional and two-dimensional displays.
Next will be described the lenticular lens type. The lenticular lens type was invented in or around 1910 by Ives and others as described in, for instance, Chihiro Masuda, Three-Dimensional Display, Sangyo Tosho Kabushiki Kaisha, p. 1 (Reference 1). FIG. 13 shows a perspective view of a lenticular lens, and FIG. 14 is an optical model diagram illustrating a three-dimensional display method by the lenticular lens type. As shown in FIG. 13, one face of a lenticular lens 121 is planar, and a plurality of convex semicircular cylindrical lenses 122, extending in one direction, are formed in parallel to one another on the other face.
Then, as shown in FIG. 14, in a three-dimensional image display device of the lenticular lens type, there are arranged the lenticular lens 121, a display panel 106 and the light source 108 in the order away from the viewer, and pixels of the display panel 106 are positioned on the focal plane of the lenticular lens 121. On the display panel 106, the pixels 123 for displaying the image for the right eye 141 and the pixels 124 for displaying that for the left eye 142 are alternately arranged. In this arrangement each group consisting of a pixel 123 and a pixel 124 adjoining each other matches one or another of the cylindrical lenses (convexes) 122 of the lenticular lens 121. This arrangement enables lights emitted from the light source 108 and having passed the pixels to be divided by the cylindrical lenses 122 of the lenticular lens 121 into the directions toward the right and left eyes and enables the right and left eyes to perceive different images. The viewer is thereby enabled to perceive a three-dimensional image. The system by which the viewer is enabled to perceive a three-dimensional image by displaying an image for the right eye and another for the left eye is known as a two-viewpoint system because the formation of two points of view are involved.
Next will be described in detail the size of each part of the three-dimensional image display device equipped with a conventional lenticular lens and a display panel. FIG. 15 is an optical model diagram of the three-dimensional image display device equipped with the conventional lenticular lens type, and FIG. 16 is an optical model diagram illustrating the three-dimensional visible area of this three-dimensional image display device.
As shown in FIG. 15, the distance between the vertex of the lenticular lens 121 and the pixels of the display panel 106 is represented by H, the refractive index of the lenticular lens 121 by n, the focal distance by f, and the array cycle of lens elements, namely the lens pitch, by L. Display pixels of the display panel 106 are arranged in a form of pairing one each of the left-eye pixel 124 and the right-eye pixel 123. The pitch of these pixels is represented by P.
Therefore, the array pitch of display pixels, of which each pair consists of one left-eye pixel 124 and one right-eye pixel 123 is 2P. One cylindrical lens 122 is arranged to match each pair of these one display pixels, consisting of one left-eye pixel 124 and one right-eye pixel 123.
The distance between the lenticular lens 121 and the viewer is supposed to be the optimal viewing distance OD, the expanded projection width of pixels at this distance OD, namely the width of each of the respective projected images of the left-eye pixels 124 and the right-eye pixels 123 on an imaginary plane at the distance OD from and parallel to the lens, is represented by e.
Further, the distance from the center of the cylindrical lens 122 positioned at the center of the lenticular lens 121 to the center of the cylindrical lens 122 positioned at an end of the lenticular lens 121 in the horizontal direction 112 is represented by WL, and the display panel 102, and the distance between the center of the paired display pixels consisting of a left-eye pixel 124 and a right-eye pixel 123 and the center of the display pixels positioned at an end of the display panel 106 in the lens array direction 112 is represented by WP. Then, the angle of incidence and the angle of emission of light at the cylindrical lens 122 positioned at the center of the lenticular lens 121 are represented by α and β, respectively, and the angle of incidence and the angle of emission of the cylindrical lens 122 at an end of the lenticular lens 121 in the lens array direction 112 are represented by γ and δ, respectively. Further the difference between the distances WL and WP is represented by C, and the number of pixels contained in the area of the distance WP, by 2 m.
Since the array cycle L of the cylindrical lenses 122 and the array cycle P of pixels are correlated, one is the basis of determining the other, but the lenticular lens is often designed to match the display panel, the array cycle P of pixels is treated as a constant. The refractive index n is determined by selecting a material for the lenticular lens 121. Unlike these factors, the viewing distance OD between the lens and the viewer and the expanded projection width of pixels e at the viewing distance OD are set to desired values. These values are used in determining the distance H between the lens vertex and the pixels and the lens pitch L. According to the Snell laws of refraction and geometrical relationships, the following Formulas 1 through 6 hold.n×sin α=sin β  (Formula 1)OD×tan β=e  (Formula 2)H×tan α=P  (Formula 3)n×sin γ=sin δ  (Formula 4)H×tan γ=C  (Formula 5)OD×tan δ=WL  (Formula 6)The following Formulas 7 through 9 also hold.WP−WL=C  (Formula 7)WP=2×m×P  (Formula 8)WL=m×L  (Formula 9)
From Formulas 1 through 3 above derive the following Formulas 10 through 12, respectively.β=arc tan(e/OD)  (Formula 10)α=acr sin(1/n×sin β)  (Formula 11)H=P/tan α  (Formula 12)
From Formula 6 and Formula 9 above derives the following Formula 13.δ=arc tan(mL/OD)  (Formula 13)
Further from Formula 7 and Formula 8 above derives the following Formula 14.C=2×m×P−m×L  (Formula 14)
Further from Formula 5 above derives the following Formula 15.δ=arc tan(C/H)  (Formula 15)
Incidentally, as the distance H between the vertex of the lenticular lens and the pixels is usually equalized to the focal distance f of the lenticular lens, the following Formula 16 holds, and the radius of curvature of the lens, represented here by r, is figured out by the following Formula 17.f=H  (Formula 16)r=H×(n−1)/n  (Formula 17)
As shown in FIG. 16, the area in which every light from the right-eye pixels 123 reaches is defined as the right-eye area 171, and the area in which every light from the left-eye pixels 124 reaches, as the left-eye area 172. If the viewer positions his right eye 141 in the right-eye area 171 and his left eye 142 in the left-eye area 172, then he can perceive a three-dimensional image.
However, as the distance between the viewer's two eyes is fixed, the right eye 141 and the left eye 142 cannot be positioned in every desired position in the right-eye area 171 and the left-eye area 172, respectively, but the visible range of the two eyes is limited to where the distance between the two eyes can be kept constant. Thus, only when the middle point between the right eye 141 and the left eye 142 is positioned in the three-dimensional visible area 107, is three-dimensional viewing possible. In the position where the distance from the three-dimensional image display device is equal to the optimal viewing distance OD, the length along the horizontal direction 112 in the three-dimensional visible area 107 is the longest, and therefore the tolerance for the deviation of the viewer's position in the horizontal direction 112 is the greatest here. For this reason, the position where the distance from the three-dimensional image display device is equal to the optimal viewing distance OD is the ideal position of observation.
While the parallax barrier system previously described “conceals” unnecessary lights with the barrier, the lenticular lens system changes the traveling direction of lights, and by its very principle is free from any decrease in the brightness of the display screen due to the presence of the lenticular lens. For this reason, it is considered to have good prospects for application to portable apparatuses, whose requirements for high luminance displaying and low power consumption are particularly stringent.
A three-dimensional image display device developed by using the lenticular lens type is described in Reference 2 cited above. The liquid crystal display panel constituting this three-dimensional image display device measures seven inches in diagonal length, and has 800 display dots horizontally and 480 vertically. By varying the distance between the lenticular lens and the liquid crystal display panel by 0.6 mm, switch-over between three-dimensional and two-dimensional displays can be accomplished.
As another example of an image display device capable of displaying different images toward a plurality of points of view, a device simultaneously displaying a plurality of images is disclosed (see the Japanese Patent Application Laid-Open No. Hei 6-332354 (see FIG. 9 thereof)). The display simultaneously displays two-dimensional images, differing from one viewing direction to another, in the same conditions by utilizing the image portioning-out function of the lenticular lens, and thereby enables a plurality of different viewers to watch at the same time different two-dimensional images in respectively different directions with a single display device.
FIG. 17 shows a perspective view of this simultaneous display of a plurality of images. As shown in FIG. 17, in this simultaneous display of a plurality of images, the lenticular lens 121 and the display panel 106 are arranged in the direction away from the viewer 104. On the display panel 106, pixels 125 for a first point of view to display an image for a first point of view and pixels 126 for a second point of view to display an image for a second point of view are alternately arranged. In this arrangement each group consisting of a pixel 125 and a pixel 126 adjoining each other matches one or another of the cylindrical lenses (convexes) 122 of the lenticular lens 121. As this arrangement enables lights transmitted through the pixels to be divided by the cylindrical lenses 122 of the lenticular lens 121 into different directions, the viewers can perceive different images in different positions. The use of this simultaneous display of a plurality of images can save an installation space, electric power charge and so forth compared with the installation of as many display devices as the viewers.
On the other hand, liquid crystal display devices, by virtue of their low power consumption and other advantages, find especially extensive use in smaller-size items including portable terminals. A liquid crystal display panel requires some external light source because it is a non-self-luminescent type, which displays an image by modulating external lights. In a common transmissive liquid crystal display panel is equipped with illuminating means, known as a backlight unit, on the rear side of the liquid crystal display panel as seen from the viewer's side (see Akira Tanaka, “The latest trend of backlights for liquid crystals”, Monthly Display, June 1997; p. 75 (Reference 3)).
FIG. 1 in Reference 3 illustrates the structure of a backlight unit for liquid crystal panel use. Usually, a backlight unit is configured of a light guiding plate for propagating lights from a light emitting source, the light emitting source known as an edge light (side light) arranged on a side of the light guiding plate, and an optical sheet arranged on the viewer's side of the light guiding plate. While the light emitted from the edge light propagates along the light guiding plate, part of the light is emitted toward the viewer, passes the transmissive liquid crystal display panel after being shaped by the optical sheet in terms of such optical characteristics as uniformity and angle distribution, and is incident on the viewer.
FIG. 18 is an optical model diagram illustrating a conventional three-dimensional display device using a lenticular lens. As described in Reference 3, a prism sheet or a lens sheet, respectively consisting of many prisms or lenses, is frequently used as the optical sheet for the backlight unit. As shown in FIG. 18, on the surface of such a prism sheet or lens sheet, there are convexes or concaves deriving from the structure of the prisms or lenses.
However, the examples of the prior art described above involve problems. A portable terminal device is required to be thin to enhance their portability, and accordingly image display devices to be mounted on the portable terminal devices are also required to be thin.